It has long been the desire of machine tool producers to provide systems which closely detect tool wear such as the edge-deterioration of a cutting tool. A deteriorated edge produces poor workpiece quality, adds more heat to the process, and draws more power for the operation of the tool. Deteriorated tool edges are often more susceptible to both chipping and chip welding. Chip packing between individual teeth may also be more prone to occur as wear progresses at the cutting edge. Any of these conditions can lead to the catastrophic failure of the tool and the mutilation of the workpiece at the time of the failure. Serious damage to machine spindle bearings and perishable tooling can also result from such events requiring significant machine down-time and expense to effect repair.
With the advent of the use of coatings (e.g. TiN, TiAIN) for super high-speed steel, solid carbide and ceramic-metal (cermet) cutting tools in the high-volume production of toothed articles, such as gear tooth generation by hobbing to produce spur and helical gears, splines and wormwheels, there is a need to closely monitor the condition of the tool to determine the wear of the coating. If a coating is completely worn through, damage can easily occur to the tool substrate resulting in a significant reduction in total tool life. With premium coated tools like those made from solid carbide, visual inspection is difficult given the low desirable edge wear limit, e.g. 0.004 inch (0.01 mm). To continually remove a tool to inspect under magnification results in considerable machine down-time, process equilibrium disturbance and increased machining costs. On the other hand, it is not cost efficient to replace a tool until it approaches its targeted wear limit.
One method of detecting tool wear has been to monitor the power consumed by the tool driveline of the machine tool. As a tool wears, more resistance to cutting is encountered and thus, more power is required to drive the tool. Increasing tool driveline power consumption is monitored and when a predetermined level is reached, which is indicative of an undesirable state of tool wear, the tool is removed for sharpening or replacement. Examples of monitoring power consumption are shown in U.S. Pat. Nos. 3,809,870 to Auble et al. and 4,442,494 to Fromson et al.
These types of systems can be quite adequate for detecting fairly gross deterioration of cutting edges and for detecting actual tool failure. However, because large inrushes of drive current are required to accelerate and decelerate the tool, it is typically necessary to set the warning limit at a threshold significantly higher than would be desirable in order to avoid nuisance interruptions in the process. Even if time intervals of tool acceleration and deceleration are ignored, it is often the case that variable cutting speeds are applied in order to lessen the peak cutting load on the tool and this feature also requires some leeway in threshold setting. Cutting loads and driveline efficiency changes at various speeds must also be factored in when determining a threshold setting.
Another method of detecting tool wear has been to provide vibration sensing systems which monitor the vibration of the tool during machining. Examples of this method are shown in U.S. Pat. Nos. 4,563,897 to Moore, 4,758,964 to Bittner et al. and 4,894,644 to Thomas. A shortcoming with vibration measurements based means is that these systems are only as reliable as the structural integrity of the machine. That is, vibrational systems are affected by harmonic responses in the driveline and structural elements, and by the mounting integrity of both the tool and workpiece spindles and supports. Furthermore, even small changes in speeds and feeds have complex effects on machining vibrations themselves, making it difficult to provide a reliable system.
Still another type of tool wear detecting system applies strain gauges to the outer races of spindle support bearings adjacent the cutting tool in an attempt to detect increases in radial and axial cutting forces which correlate with edge condition of the tool. However, preload conditions of spindle support bearings can change during the machine warm-up cycle, because of varying spindle rotational speeds or due to the presence or absence of cutting fluids. These changes diminish the reliability of data collected by the strain gauges and although special bearings and data processing equipment are available to lessen the effects of the changes, the significant costs involved result in an undesirable ultimate cost of the machine.
Yet another type of strain gauge detecting system involves applying the strain gauge equipment to the tool mounting arbor itself to measure torsional wind-up as an indicator of tool wear. Because of the constant rotation of the tool, strain gauge data is transmitted by radio signal to a receiver. While this system is successful within fairly broad tolerances, the system is highly vulnerable to the adverse conditions of machining and increases the cost of the tool mounting equipment substantially.
Still other tool wear detecting systems have been proposed which measure the axial displacement of the tool such as shown in U.S. Pat. Nos. 5,084,827 to Demesy et al. and 5,086,590 to Athanasiou. Monitoring tool wear by detecting power consumption and machine vibration is disclosed in U.S. Pat. No. 5,070,655 to Aggarwal. Monitoring tool wear by measuring the electrical resistance based on the temperature of the tool is taught in U.S. Pat. No. 5,000,036 to Yellowley et al. and by measuring the electrical resistance between the tool and workpiece is disclosed in U.S. Pat. No. 5,030,920 to Nakamura. Finally, a non-contact method of monitoring tool wear by inductively sensing the relative position of two machine parts is shown in U.S. Pat. No. 4,770,568 to Perez et al.
With all the above tool wear measuring systems, aspects of the machining process (e.g. power fluctuations, bearing load variations, exposure to machining environment, etc.) provide obstacles to proper and accurate performance of the particular measuring system. There remains a need for a tool wear monitoring system where influences of both the machining process and ambient effects are essentially nonexistent.
It is an object of the present invention to provide a method which detects the normal, progressive deterioration of the working edges of a cutting or grinding tool in order to terminate operations at a predetermined limit of edge wear.
It is another object of the present invention to provide a method which detects relatively rapid, abnormal deterioration of the working edges of the cutting or grinding tool in order to discontinue machining operations before further damage occurs.
It is yet another object of the present invention to provide a method that is independent of the vigilance of the machine operator and also sufficiently sensitive to distinguish between small amounts of tool edge wear which may not be visible to the machine operator.